In recent years, the application of silicon nitride (Si3N4) substrates to semiconductor circuit boards is being attempted. Alumina (Al2O3) substrates and aluminum nitride (AlN) substrates are used as semiconductor circuit boards. Although an alumina substrate has a thermal conductivity of around 30 W/m·K, the use thereof enables a reduction in costs. Further, in the case of an aluminum nitride substrate, it is possible to achieve high thermal conductivity in which the thermal conductivity becomes 160 W/m·K or more. On the other hand, substrates having a thermal conductivity of 50 W/m·K or more are being developed as silicon nitride substrates.
Although a silicon nitride substrate has a low thermal conductivity compared to an aluminum nitride substrate, the silicon nitride substrate has a superior three-point bending strength of 500 MPa or higher. The three-point bending strength of an aluminum nitride substrate is normally between around 300 and 400 MPa, and there is a tendency for the strength to decrease as the thermal conductivity increases. It is possible to make use of this advantage of high strength to reduce the thickness of a silicon nitride substrate. Since it is possible to lower the thermal resistance by thinning the substrate, heat dissipation (heat radiating property) is improved.
A silicon nitride substrate that makes uses of such characteristics has been widely used as a circuit board by providing a circuit portion such as a metallic plate thereon. Further, a method is also available in which a silicon nitride substrate is used as a circuit board for a pressure-contact structure, as described in International Publication No. WO 2011/010597 (Patent Document 1).
Thermal conductivity, strength and also insulating properties can be mentioned as examples of characteristics required of a silicon nitride substrate for the various usages described above.
A silicon nitride substrate with favorable insulating properties is proposed in Japanese Patent Laid-Open No. 2002-201075 (Patent Document 2). In Patent Document 2, a silicon nitride substrate is disclosed in which a current leakage value is 1000 nA or less when an alternating voltage of 1.5 kV and 100 Hz is applied between a front surface and a rear surface of the silicon nitride substrate under conditions of a temperature of 25° C. and a humidity of 70%. A lower value for the current leakage value indicates higher insulating properties between the front and rear surfaces.